II-VI laser diodes with short-period strained-layer superlattice quantum wells

ABSTRACT

A laser diode for emitting a coherent beam of light in the blue and/or green portions of the spectrum. The laser diode includes a plurality of layers of II-VI semiconductor forming a pn junction, including at least a first light-guiding layer. A short-period strained-layer superlattice (SPSLS) CdZnSe quantum well active layer is positioned within the pn junction. The layers of II-VI semiconductor are supported by a substrate. First and second electrodes on opposite sides of the layers of II-VI semiconductor couple electrical energy to the laser diode.

REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.08/139,614, filed Oct. 20, 1993 and entitled II-VI Laser Diodes WithQuantum Wells Grown By Atomic Layer Epitaxy And Migration EnhancedEpitaxy now U.S. Pat. No. 5,395,791, which is a continuation ofabandoned U.S. patent application 07/887,541, filed May 22, 1992 andentitled II-VI Laser Diodes With Quantum Wells Grown By Atomic LayerEpitaxy And Migration Enhanced Epitaxy now abandoned.

BACKGROUND OF THE INVENTION

Research undertaken by 3M in St. Paul, Minn., culminated in thedemonstration of the world's first laser diodes fabricated from II-VIsemiconductor materials. These devices emit coherent radiation at 490 nmin the blue-green portion of the spectrum. They are disclosed generallyin Haase et al. article, Short Wavelength II-VI Laser Diodes, ConferenceProceedings for Gallium Arsenide and Related Compounds, 1991 Instituteof Physics Conference Series, No. 120, pp 9-16.

The light-generating (active) layers in the above-described laser diodesinclude strained Cd_(x) Zn_(1-x) Se single quantum wells grown byconventional molecular beam epitaxy (MBE) techniques. Unfortunately, thecomposition and thickness of the random CdZnSe alloy is difficult tocontrol by this process. Luminescence efficiency is also relativelypoor. These characteristics limit the overall efficiency of the devices.

It is evident that there is a continuing need for improved laser diodes.For wide spread commercial viability, the devices must be capable ofefficiently generating high intensity beams of light at roomtemperatures. Fabrication techniques for laser diodes having thesecharacteristics are also needed.

SUMMARY OF THE INVENTION

The present invention is a II-VI compound semiconductorelectroluminescent device capable of efficiently generating highintensity beams of light. One embodiment of the device includes aplurality of layers of II-VI semiconductor forming a pn junction, and ashort-period strained-layer superlattice (SPSLS) active layer within thepn junction. The SPSLS active layer has a form described by the notation[(X)_(m) (Y)_(n) ]_(p), where X and Y are binary II-VI semiconductorcompounds and m, n and p are integers. The layers of II-VI semiconductorare supported by a substrate. Electrical energy is coupled to the deviceby first and second electrodes on opposite sides of the pn junction.

In another embodiment, the device is a laser diode with an SPSLS quantumwell active layer. The SPSLS quantum well layer can be of a formdescribed by the notations [(CdSe)_(m) (ZnSe)_(n) ]_(p), [(ZnTe)_(m)(ZnSe)_(n) ]_(p) and [(ZnSe)_(m) ]_(p), where m, n and p are integers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view (not to scale) illustrating thestructure of a II-VI semiconductor laser diode in accordance with thepresent invention.

FIG. 2 is a graph illustrating the product of the loss coefficient (α)and the full width at half maximum (FWHM) intensity of the optical modeas a function of the thickness of the light-guiding layers, for laserdiodes of the type shown in FIG. 1.

FIG. 3 is an illustration of a molecular beam epitaxy (MBE) system thatcan be used to fabricate laser diodes in accordance with the presentinvention.

FIG. 4 is a detailed cross sectional view of the quantum well layershown in FIG. 1.

FIG. 5 is a graph of the shutter sequence by which the MBE system, shownin FIG. 3, is operated to fabricate the active layer of laser diodes inaccordance with the present invention.

FIG. 6 is a high resolution transmission electron micrograph of across-section of the quantum well of a laser diode fabricated inaccordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A II-VI compound semiconductor laser diode 10 (i.e., anelectroluminescent device) in accordance with the present invention isillustrated generally in FIG. 1. Laser diode 10 includes a short-periodstrained-layer superlattice (SPSLS) quantum well layer 12 surrounded bya ZnSe pn junction formed by n-type ZnSe light-guiding layer 14 andp-type ZnSe light-guiding layer 16. As described in greater detailbelow, quantum well layer 12 is a high efficiency active layer grown byatomic layer epitaxy (ALE) and/or migration enhanced epitaxy (MEE).Laser diode 10 is fabricated on an n-type GaAs substrate 18 and includesan n-type ZnSe ohmic contact layer 19 between the substrate and guidinglayer 14. A p-type ZnSe ohmic contact layer 20 overlays p-type guidinglayer 16. A polyimide insulating layer 22 covers the surface of ohmiccontact layer 20 opposite light-guiding layer 16.

Electrical contact to p-type ohmic contact layer 20 is made by Auelectrode 24, which is formed in an open stripe in insulating layer 22.A thin Ti layer 26 and a final Au layer 28 are applied over insulatinglayer 22 to facilitate lead bonding. Electrical contact to the lowerside of laser diode 10 is made by an In electrode 30 on the surface ofsubstrate 18 opposite n-type ohmic contact layer 19.

Light-guiding layer 14 and contact layer 19 are both doped n-type withCl in prototypes of laser diode 10. Light-guiding layer 16 and ohmiccontact layer 20 are doped p-type with N in these prototypes. The netdonor concentration to which the lower light-guiding layer 14 is dopedis 1×10¹⁷ cm⁻³, while the upper light-guiding layer 16 is doped to a netacceptor concentration of 2×10¹⁷ cm⁻³. Ohmic contact layers 19 and 20are both deposited to a thickness of 0.1 μm in the prototype devices.The lower contact layer 19 is doped n-type to a net donor concentrationof 1×10¹⁸ cm⁻³. The upper contact layer 20 is doped p-type to a netacceptor concentration of 1×10¹⁸ cm⁻³.

Light generated in quantum well active layer 12 is guided withinlight-guiding layers 14 and 16, clad only by the GaAs substrate 18 andthe Au electrode 24. Good optical confinement and sufficiently low lossare obtained in laser diode 10 without the need for II-VI semiconductorcladding layers. Computer modeling is used to select appropriatethicknesses for light-guiding layers 14 and 16. This modeling approachtakes into account the ZnSe waveguide formed by light-guiding layers 14and 16, as well as the complex indices of refraction of GaAs substrate18 and Au electrode 24. Modeling methods of this type are generallyknown and disclosed, for example, in M. R. Ramdas et al., Analysis ofAbsorbing and Leaky Planar Waveguides: A Novel Method, Optics Letters,Vol. 14, p. 376 (1989) and the references cited therein.

FIG. 2 is a graph illustrating the product of the loss coefficient (α)and the full width at half maximum intensity (FWHM) of the desiredoptical mode (TE polarized for the prototypes described herein) as afunction of the thickness of the ZnSe layers 14 and 16 (FIG. 1). Tominimize the threshold current density of the device, this productshould be minimized. Using this design criterion and the informationshown in FIG. 2, the thickness of the waveguide (i.e., the combinedthicknesses of light-guiding layers 14 and 16) is approximately 3.5 μmin the prototype laser diode 10. In this embodiment, n-typelight-guiding layer 14 has a thickness of 2.0 μm, while p-typelight-guiding layer 16 has a thickness of 1.5 μm. The loss due tofree-carrier absorption and scattering is estimated to be 8 cm⁻¹ in thisembodiment. Quantum well layer 12 has only a relatively small effect onthe loss and optical confinement characteristics of the device, and itspresence is neglected during the design procedure described above.Theory suggests that total waveguide thicknesses less than 2.0 μm resultin excessive absorption losses in substrate 18 and electrode 24. At athickness of 2.5 μm, the substrate and electrode absorption losses are11.7 cm⁻¹. On the other hand, the FWHM of the optical mode is found tobe almost exactly half of the waveguide thickness. Therefore, forthicknesses greater than about 6 μm the optical confinement is so poorthat the single quantum well layer 12 cannot practically supply enoughgain to overcome the losses. The maximum modal gain is inverselyproportional to the FWHM of the waveguide mode. For a waveguide that is6 μm thick, the FWHM is about 3 μm, and the modal gain from a singlequantum well can be estimated to be 12 cm⁻¹. See, e.g., N. K. Dutta,Applied Physics Letters, vol. 53, p. 72 (Nov. 1982).

FIG. 3 is an illustration of a molecular beam epitaxy (MBE) system usedto fabricate the prototypes of laser diode 10 described above. The MBEsystem includes a chamber 54 with a high energy electron gun 58, aphosphorus screen 60, a substrate heater 90 and a flux monitor 62. MBEchambers such as 54 are generally known and commercially available.

Prototypes of laser diode 10 are fabricated on Si-doped N⁺ -type GaAssubstrates 18 having a (100) crystal orientation. Substrates of thistype are commercially available. Substrate 12 is cleaned and preparedusing conventional or otherwise known techniques, and mounted to aMolybdenum sample block (not shown in FIG. 3) by In solder before beingpositioned within chamber 54.

Chamber 54 includes a Zn effusion cell 70, cracked-Se effusion cell 72,Cd effusion cell 76 and a standard Se (i.e., primarily Se₆) effusioncell 79. As shown, cracked-Se effusion cell 72 includes a bulkevaporator 84 and high temperature cracking zone 82, and provide asource of cracked Se (including Se₂ and other Se molecules with lessthan 6 atoms). The bulk evaporator 84 and high temperature cracking zone82 used to produce the prototype laser diodes 10 are of a custom design,the details and capabilities of which are described in the Cheng et al.article, Molecular-Beam Epitaxy Growth of ZnSe Using a Cracked SeleniumSource, J. Vac. Sci. Technol., B8, 181 (1990). Cl effusion cell 78 whichutilizes ZnCl₂ source material provides the Cl n-type dopant. The p-typedopant is provided by N free-radical source 80. Free-radical source 80is connected to a source 86 of ultra-pure N₂ through leak-valve 88. Thefree-radical source 80 used in the fabrication of laser diodes 10 iscommercially available from Oxford Applied Research Ltd. of Oxfordshire,England (Model No. MPD21). The beam exit plate at the end of the sourceis made of pyrolyric boron nitride (PBN) and has nine 0.2 mm diameterholes through it. This source is mounted on a standard port for aneffusion cell through a 10" extension tube. N₂ source 86 used tofabricate laser diodes 10 is of research purity grade. The pressure atthe inlet of the leak-valve of source 86 is 5 psi.

MBE chamber 54 is operated in a manner described in the Cheng et al.article Growth of p- and n-type ZnSe by Molecular Beam Epitaxy, J.Crystal Growth 95, 512 (1989) using the Se₆ source 79 as the source ofSe to grow the n-type contact and light-guiding layers 19 and 14,respectively, of the prototype laser diode 10.

SPSLS quantum well layer 12 is grown on the light-guiding layer 14 oflaser diode 10 using atomic layer epitaxy (ALE) and/or migrationenhanced epitaxy (MEE). Using these techniques, which are generallyknown, quantum well layer 12 is formed as a series of overlaying singlecrystal thickness layers (i.e., monolayers) of Cd, Zn and Se. A detailedillustration of quantum well layer 12 is shown in FIG. 4. In thisembodiment, quantum well layer 12 includes adjacent monolayers of Cd andSe between a pair of adjacent monolayers of Zn and Se. This structurecan be described generally by the following notation:

[(CdSe)_(m) (ZnSe)_(a) ]_(p)

where: m, n and p are integers.

In the embodiment illustrated in FIG. 4, m=1, n=2, and p=1. In otherembodiments (not shown) m=1, n=1-5 and p=1-5. The equivalent Cdconcentration in quantum well layer 12 is determined by the ratio of thenumber of CdSe layers to the total number of layers (including both ZnSeand CdSe) in the quantum well layer. Total thickness of quantum welllayer 12 is given by the number of monolayers grown times the thicknessof each monolayer. FIG. 6 is a high resolution transmission electronmicrograph analysis of a cross section of a prototype laser diode 10having a SPSLS quantum well with a period (p) equal to 6, clearlyrepresenting the monolayer structure of the quantum well.

Control over the composition and thickness of the Cd, Zn and Semonolayers of quantum well layer 12 are accurately achieved by ALEand/or MEE. Through use of these techniques, monolayer growth iscontrolled primarily by the sequence and timing by which the shutters(not separately shown) of the Cd, Zn and Se effusion cells 76, 70, and72, respectively, are opened and closed. A portion of the effusion cellshutter sequence used to grow the quantum well layer 12 illustrated inFIG. 4 is shown in FIG. 5. A characteristic delay time is introducedbetween the sequential pulses of the reactant species to allow for thereevaporation of excess reactant.

Prototype laser diodes 10 including SPSLS quantum well layers such asthat shown in FIG. 4 have been grown at temperatures of 150° C. andusing the thermally cracked Se (Se₂) effusion cell 76. The shuttersequence begins with the Se shutter open. The Se shutter is closed afterdepositing at least one monolayer (about 5 seconds). The Zn shutter isthen opened after a slight delay (about 2 seconds) to allow excess Se toevaporate. Next, the Zn shutter is closed after depositing at least onemonolayer of Zn (about 4 seconds). A slight delay (about 1 second) isincluded between the closing of the Zn shutter and the reopening of theSe shutter to allow time for the evaporation of any excess Zn. Growthcontinues by alternately opening and closing the shutters tosequentially deposit overlaying layers of Cd, Se and Zn. The Cd shutteris opened for about 4 seconds, followed by a delay of about 1 secondbefore the Se shutter is reopened. The sequence beginning with the openSe shutter is then repeated to complete quantum well layer 12. Otheroperating parameters of MBE chamber 54 used to produce the quantum welllayer 12 of the prototype laser diodes 10 are as follows:

Cd beam equivalent pressure: 1.0×10⁻⁷ Torr*

Zn beam equivalent pressure: 1.0×10⁻⁷ Torr*

Se cracking zone temperature: 600° C.*

Se bulk evaporator temperature: 250° C.*

* parameters dependent upon specific MBE system configuration and plasmasource

Prototype laser diodes 10 having SPSLS quantum well layers 12 grown inthe manner described above at 150° C. have exhibited the highest quantumefficiency. However, quantum well test wafers with SPSLS quantum welllayers grown at temperatures up to 235° C. have exhibited goodcharacteristics. Acceptable characteristics have been observed in testwafers with SPSLS quantum well layers grown at temperatures as high as300° C. It is also anticipated that devices having desirablecharacteristics can be grown at temperatures less than 150° C.

MBE chamber 54 is operated in a manner described in the Park et al. U.S.Pat. No. 5,248,631 entitled Doping of IIB-VIA Semiconductors DuringMolecular Beam Epitaxy Using Neutral Free Radicals, and in the Park etal. article, P-type ZnSe by Nitrogen Atom Beam Doping During MolecularBeam Epitaxial Growth, Appl. Phys. Lett. 57, 2127 (1990), using the Se₆source 79 to grow the p-type light-guiding layer 16. The disclosurescontained in the above-referenced Park et al. U.S. Patent and articleare incorporated herein by reference.

Low resistivity p-type ZnSe ohmic contact layer 20 has been achieved bygrowing the contact layer at low temperature within MBE chamber 54utilizing the cracked Se source 72 (i.e., cracking zone 82 andevaporator 84), while at the same time doping the semiconductor materialof the contact layer p-type. This low temperature growth and dopingtechnique is described in greater detail in the DePuydt et al. U.S. Pat.No. 5,274,269. The semiconductor body with layers 19, 14, 12, and 16 onsubstrate 18 is heated to a temperature less than 250° C. but highenough to promote crystalline growth of the ZnSe doped with the N p-typedopants to a net acceptor concentration of at least 1×10¹⁷ cm⁻³. A netacceptor concentration of 1-10¹⁸ cm⁻³ was achieved in the ohmic contactlayer 20 of prototype laser diodes 10, when grown at a substratetemperature of about 150° C. However, it is anticipated that ohmiccontact layers 20 with acceptable characteristics can be achieved atother growth temperatures down to at least 130° C. Other operatingparameters of MBE chamber 54 used to produce the ohmic contact layer 20of the prototype laser diodes 10 are as follows:

Zn beam equivalent pressure: 1.0-10⁻⁷ Torr*

Se cracking zone temperature: 600° C.*

Se bulk evaporator temperature: 250° C.*

Growth rate: 0.3-0.6 μm/hr

Surface reconstruction: Zn-stabilized

Nitrogen pressure in chamber: >3.5×10⁻⁷ Torr*

rf power: 150-250 W *

* parameters dependent upon specific MBE system configuration

Following the deposition of contact layer 20, the as yet incompletelaser diode 10 is removed from MBE chamber 54. Electrode 24 includes Auwhich is vacuum evaporated onto contact layer 20 and patterned into astripe (typically about 20 μm wide) using conventional photolithographyand lift-off. An insulating layer 22 is then applied over electrode 24and the exposed surface of contact layer 20. For an insulator that canbe applied at low temperatures, polyimide photoresist is preferred.Probimide 408 from Ciba-Geigy Corp. was used to produce the prototypelaser diodes 10. A stripe (about 20 μm wide) of the polyimide layer 22directly above electrode 24 is removed by UV exposure through aphotomask and development using the manufacturer's recommendedprocessing recipe, except for the post-development cure. To cure thedeveloped polyimide, the device was flood exposed to 1 J/cm² of UV lightfrom a mask aligner, and baked at 125° C. on a hot plate in air for 3minutes. Ti-Au layer 26 was then evaporated on the exposed surface ofthe Au electrode 24 and polyimide layer 22 to facilitate lead-bonding.The In used for MBE substrate bonding also served as electrode 30 onsubstrate 18. Opposite ends of the device were cleaved along (110)planes to form facet mirrors. The facets were coated with a total offour alternating quarter wavelength stacks of MgF₂ and ZnSe to provide a90% reflectivity. Cavity length of the prototype devices 10 is about1000 μm. Laser diodes 10 were then bonded p-side up to ceramic sampleholders with silver-filled epoxy.

Laser diodes 10 in accordance with the present invention offerconsiderable advantages. The ALE/MEE techniques for growing the quantumwell layer enables better composition control, better control of thequantum well thickness and luminescence efficiency than random alloyquantum wells grown by conventional MBE. These characteristics areevidenced by increased room temperature photoluminescence andelectroluminescence intensities, and lower laser threshold currents.Although the room temperature threshold currents in these prototypedevices (as low as 1030 A/cm³) are not as low as in devices withcladding layers, these devices offer the advantages of simplerconstruction and lower operating voltages (about 13 V).

The embodiments described above include light guiding layers that arenot lattice matched to the substrate. It is anticipated that increaseddevice lifetime and improved performance will be exhibited by devices inwhich the lattice parameters of the light guiding layers are matched tothat of the substrate. The first and second light guiding layer mayinclude ZnSo₀.06 Se₀.94 or Cd₀.43 Zn₀.57 S on GaAs substrates or ZnSelight guiding layers on In₀.04 Ga₀.96 As or In₀.52 Ga₀.48 P substrates.The light guiding layers may also be comprised of ZnS_(x) Se_(1-x),Cd_(x) Zn_(1-x) S, ZnS_(1-x) Te_(x), Zn_(1-x) Cd_(x) Se, Zn_(1-x) Mg_(x)S_(y) Se_(1-y) or Cd_(x) Zn_(1-Y) Mg_(y) S layers lattice matched tosubstrates such as GaAs, AlAs, GaP, Al_(x) Ga_(1-x) As, In_(x) Ga_(1-x)As, In_(x) Al_(1-x) As, In_(x) Ga_(1-xP), In_(x) Al_(1-x) P, GaAs_(1-x)P_(x), In_(x) Ga_(1-x-y) Al_(y) As, In_(x) Ga_(1-x-y) Al_(y) P, ZnSe orZn_(1-x) Cd_(x) S.

Although the present invention has been described with reference topreferred embodiments, those skilled in the art will recognize thatchanges may be made in form and detail without departing from the spiritand scope of the invention. For example, it is expected that theinventive concepts used to fabricate the prototype laser diodesdisclosed herein are well suited to the fabrication of laser diodes withALE/MEE active layers and/or guiding layers from a wide variety of otherII-VI semiconductors and alloys. These include ZnSe, ZnTe, ZnSeTe, CdS,CdZnSeTe, MgZnSe, CdZnS, ZnSTe and CdZnTe.

What is claimed is:
 1. A II-VI compound semiconductor electroluminescentdevice including:a plurality or layers of II-VI semiconductor forming apn junction; a short-period strained-layer superlattice (SPSLS) activelayer within the pn junction, the SPSLS layer including a plurality ofoverlaying single crystal thickness layers of elements and having a formdescribed by the notation ((X)_(m) (Y)_(n))_(p), where X and Y arebinary II-VI semiconductor compounds and m, n and p are integers; asubstrate for supporting the layers of II-VI semiconductor; and firstand second electrodes for coupling electrical energy to the device. 2.The electroluminescent device of claim 1 wherein the SPSLS active layerincludes a quantum well layer described by the notation ((CdSe)_(m)(ZnSe)_(n))_(p), where m, n and p are integers.
 3. Theelectroluminescent device of claim 1 wherein the SPSLS active layerincludes a quantum well layer described by the notation ((ZnTe)_(m)(ZnSe)_(n))_(p) where m, n and p are integers.
 4. The electroluminescentdevice of claim 1 wherein the SPSLS active layer includes a quantum welllayer described by the notation _(p) where m and p are integers.
 5. Theelectroluminescent device of claim 1 wherein the SPSLS active layerincludes a SPSLS quantum well active layer.
 6. A II-VI compoundsemiconductor laser diode, including:a plurality of layers of II-VIsemiconductor forming a pn junction; a short-period strained-layersuperlattice (SPSLS) quantum well active layer within the pn junction,the SPSLS layer including a plurality of overlaying single crystalthickness layers of elements and having a form described by the notation((X)_(m) (Y)_(n))_(p) where X and Y are binary II-VI semiconductorcompounds and m, n and p are integers; a semiconductor substrate forsupporting the layers of II-VI semiconductor; and first and secondelectrodes for coupling electrical energy to the laser diode; first andsecond electrodes for coupling electrical energy to the laser.
 7. Thelaser diode of claim 6 wherein the SPSLS active layer includes a layerdescribed by the notation ((CdSe)_(m) (ZnSe)_(n))_(p) where m, n and pare integers.
 8. The laser diode of claim 6 wherein the SPSLS activelayer includes a layer described by the notation ((ZnTe)_(m)(ZnSe)_(n))_(p), where m, n and p are integers.
 9. The laser diode ofclaim 6 wherein the SPSLS active layer includes a layer described by thenotation ((ZnSe)_(m))_(p), where m and p are integers.
 10. The laserdiode of claim 6 wherein the plurality of layers of II-VI semiconductorinclude at least a first light-guiding layer.
 11. The laser diode ofclaim 6 wherein the plurality of layers of II-VI semiconductor include:afirst light-guiding layer of II-VI semiconductor of a first conductivitytype; and a second light-guiding layer of II-VI semiconductor of asecond conductivity type adjacent to the first light-guiding layer, thefirst and second light-guiding layers forming the pn junction.
 12. Thelaser diode of claim 6 wherein the SPSLS quantum well active layerincludes a layer described by the notation ((X)_(m) (Y)_(n))_(p) where Xand Y are binary II-VI semiconductor compounds and m=1-3, n=1-5 andp=1-5.
 13. A laser diode of the type emitting a coherent beam of lightin the blue and/or green portions of the spectrum, including:a pluralityof layers of II-VI semiconductor forming a pn junction, including atleast a first light-guiding layer; a short-period strained-layersuperlattice (SPSLS) CdZnSe quantum well active layer within the pnjunction, the SPSLS layer including a plurality of overlaying singlecrystal thickness layers of Cd, Zn and Se; a semiconductor substrate forsupporting the layers of II-VI semiconductor; and first and secondelectrodes for coupling electrical energy to the laser diode.